Low pressure drop reforming exchanger

ABSTRACT

Disclosed are a syngas production process and a reforming exchanger  100 . The process involves passing a first portion of hydrocarbon feed mixed with steam and oxidant through an autothermal catalytic steam reforming zone to form a first reformed gas of reduced hydrocarbon content, passing a second portion of the hydrocarbon feed mixed with steam through an endothermic catalytic steam reforming zone to form a second reformed gas of reduced hydrocarbon content, and mixing the first and second reformed gases and passing the resulting gas mixture through a heat exchange zone for cooling the gas mixture and thereby supplying heat to the endothermic catalytic steam reforming zone. The endothermic catalytic steam reforming zone and the heat exchange zone are respectively disposed tube side and shell side within a shell-and-tube reforming exchanger  100 . The reforming exchanger  100  comprises a plurality of tubes  118  packed with low pressure drop catalyst-bearing monolithic structures  200  wherein an inside diameter of the tubes  118  is less than 4 times a maximum edge dimension of the catalyst structures  200.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 09/908,307, having a filing date of Jul. 18, 2001, now U.S.Pat. No. 6,855,272.

FIELD OF THE INVENTION

This invention relates to reforming exchangers for syngas production,and more particularly to reforming exchangers with catalyst tubes havinga relatively low ratio of inside tube diameter to catalyst particlesize.

BACKGROUND OF THE INVENTION

Steam reforming of a hydrocarbon to manufacture syngas is a very wellknown process. One popular technique is to use an autothermal reformerin conjunction with a reforming exchanger, as described in U.S. Pat. No.5,011,625 to Le Blanc, which is hereby incorporated by reference hereinin its entirety. Briefly, the hydrocarbon and an oxygen source aresupplied to the autothermal reformer. The combustion reaction isexothermic and supplies the heat needed for the catalytic reformingreaction that occurs in the autothermal reformer, which is endothermic,to produce a relatively hot reformed gas. The hot gas from theautothermal reformer is then used as a heat source in the reformingexchanger, which is operated as an endothermic catalytic steam reformingzone. In the reforming exchanger, a feed comprising a mixture of steamand hydrocarbon is passed through catalyst-filled tubes. The outlet endsof the tubes discharge the endothermically reformed gas near the shellside inlet where it mixes with the hot gas from the autothermalreformer. The hot gas mixture is then passed countercurrently across thetubes in indirect heat exchange to supply the heat necessary for theendothermic reforming reaction to occur.

Reforming exchangers are in use commercially and are available, forexample, from Kellogg Brown & Root, Inc. under the trade designationKRES. Various improvements to the reforming exchanger design have beenmade, as disclosed in, for example, U.S. Pat. No. 5,362,454 to Cizmar etal., which is hereby incorporated by reference herein in its entirety.

The present invention addresses improvements to the basic reformingexchanger design. The primary design consideration is to minimize thecapital cost of the equipment, which is complicated because expensivealloys must be used to construct the tube bundle and tube sheets for therelatively high operating temperatures and pressures. Another designconsideration is to maximize the capacity of the reforming exchangerwithin the practical limits of fabrication capabilities. It is alsodesirable to minimize the size and weight of the reforming exchanger tofacilitate maintenance operations that require removal of the tubebundle.

Our approach to reducing the capital cost and increasing the capacity ofthe reforming exchanger is to increase the heat transfer rate byincreasing the surface area available for heat transfer. Increasing thelength of the conventional catalyst-filled tubes in the existingreforming exchanger design, however, was not practical because the tubeside pressure drop (ΔP) would increase beyond that permitted withoutunduly complicating the tube sheet and tube sheet support designs, aswell as increasing upstream operating pressures and compression costs.Furthermore, longer tubes would complicate the maintenance operationsinvolving removal of the tube bundle.

The other approach to increasing the heat transfer area is to reduce thediameter of the catalyst-filled tubes. However, it was a commonly heldbelief among the reforming reactor designers that the inside diameter ofthe heat transfer tubes had to be a minimum of 5 times the diameter orother largest edge dimension of the catalyst particles, supposedlybecause of packing, bridging, flow channeling, and other potentialproblems. For example, James T. Richardson, Principles of CatalystDevelopment, Plenum Press, New York, N.Y., p. 8 (1986) (citing E. G.Christoffel, “Laboratory Reactors and Heterogeneous CatalyticProcesses,” Catal. Rev.—Sci. Eng., vol. 24, p. 159 (1982)), reports thatthe reactor to particle diameter ratio should be from five to ten, withthe reactor length at least 50-100 times the particle diameter, toensure that the flow is turbulent, uniform, and approximately plug flow.

To observe these design criteria would mean that reducing the tubediameter and increasing the number of tubes, as a means of increasingthe available surface area, would require using a smaller catalyststructure. For example, in tubes having a 2-in. inside diameter (ID),the longitudinally bored, cylindrical catalyst shapes, also known asRaschig rings, prevalent in reforming exchangers used in the art wouldtypically measure 0.315-in. outside diameter (OD) by 0.125-in. ID by0.31-in. long. When small-ID tubes were specified, it was thought thatthe size of the catalyst particles had to be correspondingly reduced toadhere to the traditional equation D_(t)/D_(p)>5, wherein D_(t) is theinside diameter of the tubes in the reforming exchanger and D_(p) is themaximum edge dimension of the catalyst structure. Unfortunately, the useof smaller catalyst particles in smaller tubes, to observe thisconventional design criterion, resulted in an unacceptable increase intube side pressure drop. Conversely, because existing reformingexchanger designs were already at or near the maximum ratio of catalystsize to tube ID, the catalyst size could not be increased in theexisting tube design as a means for reducing the pressure drop per unitof tube length so as to allow the use of longer tubes. It appeared as ifthere would be no practical way to increase the heat transfer, and thatthe ultimate capacity limits of the reforming exchanger design had beenreached.

SUMMARY OF THE INVENTION

In the investigation of the present reforming exchanger designs, it wasobserved that the endothermic catalytic reforming reaction in thereforming exchanger is limited by heat transfer and not limited bycatalyst activity. In other words, increasing the heat transfer betweenthe shell side and tube side fluids in the reforming exchanger wouldtend to increase the rate of reaction, whereas increasing or decreasingthe catalyst activity or surface area would have less effect on thereaction rate. With this observation, the present applicants are able toincrease the heat transfer coefficients by using catalyst that has arelatively low tube side pressure drop (ΔP), but does not necessarilyhave an equivalent catalytic activity or geometric surface area.Further, the present applicants discovered that by carefully packing thetubes with the catalyst particles, the conventional design relationshipbetween the catalyst diameter, D_(p), and the tube inside diameter,D_(t), is not applicable, and that a D_(t)/D_(p) ratio of preferablyless than 4, and more preferably 3 or less, is employed to reduce thepressure drop in smaller inside diameter (ID) tubes. Quite surprisingly,the present applicants also discovered that the use of smaller ID tubeswith a smaller D_(t)/D_(p) ratio results in higher heat transfercoefficients and greater efficiency than tubes designed with aconventional D_(t)/D_(p) ratio.

The present invention thus provides a solution to the dilemma in theprior art reforming exchanger designs. The present invention is thediscovery of a tube design in a syngas reforming exchanger employing acatalyst structure and/or arrangement that allows the use of relativelylong and/or relatively small-ID tubes. The catalyst-packed tubes canhave a lower D_(t)/D_(p) ratio than in the prior art reformingexchangers. This allows the capacity of the reforming exchanger to beincreased for a given size. Alternatively or additionally, the size andcost of the reforming exchanger for a given syngas production capacitycan be significantly reduced.

In one aspect, the invention provides a syngas production processcomprising: (1) passing a first portion of a hydrocarbon feed mixed withsteam and oxidant through an autothermal catalytic steam reforming zoneto form a first reformed gas of reduced hydrocarbon content; (2) passinga second portion of the hydrocarbon feed mixed with steam through anendothermic catalytic steam reforming zone to form a second reformed gasof reduced hydrocarbon content; (3) mixing the first and second reformedgases and passing the resulting gas mixture through a heat exchange zonefor cooling the gas mixture and supplying heat to the endothermiccatalytic steam reforming zone; (4) wherein the endothermic catalyticsteam reforming zone and the heat exchange zone are respectivelydisposed tube side and shell side within a shell-and-tube reformingexchanger comprising a plurality of tubes packed with catalyst-bearingmonolithic structures, wherein D_(t)/D_(p) is not more than 4, whereinD_(t) is the inside diameter of the tubes and D_(p) is a maximum edgedimension of the catalyst structures; and (5) recovering syngascomprising the cooled gas mixture.

In another aspect, the invention provides apparatus for reforming ahydrocarbon to produce syngas. The apparatus includes means for passinga first portion of a hydrocarbon feed mixed with steam and oxidantthrough an autothermal catalytic steam reforming zone to form a firstreformed gas of reduced hydrocarbon content. Means are provided forpassing a second portion of the hydrocarbon feed mixed with steamthrough an endothermic catalytic steam reforming zone to form a secondreformed gas of reduced hydrocarbon content. Means are also provided formixing the first and second reformed gases and passing the resulting gasmixture through a heat exchange zone for cooling the gas mixture andsupplying heat to the endothermic catalytic steam reforming zone. Theendothermic catalytic steam reforming zone and the heat exchange zoneare respectively disposed tube side and shell side within ashell-and-tube reforming exchanger comprising a plurality of tubespacked with catalyst-bearing monolithic structures. The tubes have aninside diameter that is not more than 4 times a maximum edge dimensionof the catalyst structures. Means are further provided for recoveringsyngas comprising the cooled gas mixture.

In a further aspect, the invention provides a syngas reformingexchanger. The exchanger has a tube side fluid inlet, a shell side fluidinlet and outlet, and an elongated shell having relatively high and lowtemperature ends. The shell side fluid inlet is adjacent to the hightemperature end for receiving a hot gas feed. The tube side fluid inletis adjacent to the low temperature end for receiving a feed mixture ofhydrocarbon and steam. The shell side fluid outlet is fluidly isolatedfrom the tube side fluid inlet by a tube sheet that is adjacent to thelow temperature end for discharging cooled gas from the reformingexchanger. A tube bundle is made up of a plurality of tubes and one ormore longitudinally spaced transverse baffle plates. The tubes have aninlet end secured to the tube sheet for receiving the feed mixture andan outlet end that is adjacent to the shell side fluid inlet fordischarging reformed gas into the hot gas feed. Catalyst-bearingmonolithic structures are disposed within the tubes for converting thegas feed mixture to reformed gas. The tubes have an inside diameter thatis not more than 4 times a maximum edge dimension of the catalyststructures.

In the process, apparatus and reforming exchanger, the tubes preferablyhave an L_(t)/D_(t) ratio of at least 300, wherein L_(t) is the lengthof the catalyst packing in the tubes. The combination of the tube ID andthe catalyst-bearing monolithic structures preferably results in ahigher heat transfer rate for the same given pressure drop, i.e. whereinan overall heat transfer rate is at least 5 percent greater for a givenpressure drop than Raschig rings measuring 0.31-in. long by 0.31-in.outside diameter by 0.125-in. inside diameter in tubes having an insidediameter of 2-in.

The catalyst-bearing monolithic structures, in one embodiment, comprisea twisted tape insert. The twisted tape insert is preferably made ofnickel or a nickel alloy, and can have a wash-coated surface impregnatedwith a nickel-containing catalyst. The twisted tape insert can have alength that is coextensive with the catalyst-packed length of the tubesand an OD that is about equal to the tube ID, such that D_(t)/D_(p) issubstantially less than 1, taking the length of the twisted tape insertas the longest edge dimension for D_(p) as defined above.

In another embodiment, the catalyst-bearing monolithic structurescomprise a central longitudinal runner and a plurality of bristlesextending transversely therefrom. The bristles can be wash-coated andimpregnated with a nickel-containing catalyst. Again, the runner canhave a length that is coextensive with the catalyst-packed length of thetubes and an OD that is about equal to the tube ID, such thatD_(t)/D_(p) is substantially less than 1, again taking the length of therunner as the longest edge dimension for D_(p) as defined above.

In a further embodiment, the catalyst-bearing monolithic structurescomprise ceramic foam. The ceramic foam can be made by filling voids inan organic sponge substrate with a fluidized ceramic precursor andburning the substrate away to form the ceramic foam. The ceramic foamcan be impregnated with nickel or another catalytically active material,and is preferably made in sheets, plugs are cut from the sheets having adiameter less than a thickness, and a plurality of the plugs are stackedend-to-end in each tube. The ceramic foam plugs preferably have a lengthor height that is about equal to or greater than the ID of the tubes andan OD that is about equal to the tube ID, such that D_(t)/D_(p) isapproximately 1.

Still further, in another embodiment, the catalyst-bearing monolithicstructures comprise finned, hollow cylinders, also called ribbed rings.The ribbed rings preferably have a longitudinal bore formed along acentral axis. The depth of channels between the fins, i.e. the radialheight of the fins, is preferably from 0.1 to 0.3 times the outsidediameter (OD) of the structures, measuring to an outer diameter of thefins. The ribbed rings can have a length that is roughly ⅓ to ¼ the tubeID, such that D_(t)/D_(p) is from about 3 to about 4.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a reforming exchanger.

FIGS. 2A and 2B are end and perspective views, respectively, of a ribbedring catalyst support according to one embodiment of the invention.

FIG. 3 is a perspective view, partially cut away, of the ribbed ringcatalyst support of FIGS. 2A and 2B packed in a tube of the reformingexchanger according to the invention.

FIG. 4 is a perspective view, partially cut away, of a twisted tapecatalyst monolith insert in a tube of the reforming exchanger accordingto another embodiment of the invention.

FIG. 5 is a perspective view, partially cut away, of a brush catalystmonolith insert in a tube of the reforming exchanger according to afurther embodiment of the invention.

FIGS. 6A and 6B are end and perspective views, respectively, of aplug-shaped ceramic foam catalyst support according to anotherembodiment of the invention.

FIG. 7 is a perspective view, partially cut away, of the ceramic foamcatalyst support of FIGS. 6A and 6B packed in a tube of the reformingexchanger according to the invention.

FIG. 8 is a bar chart comparing the heat transfer/pressure drop ratiofor ribbed ring, Raschig ring and foam catalyst supports in 1.38-in. or1.05-in. inside diameter (ID) tubes, and for ribbed rings in a 1.94-in.ID tube, in a reforming exchanger, relative to Raschig rings in a1.94-in. ID tube.

DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a reforming exchanger 100 generallybuilt according to the disclosures in the LeBlanc and Cizmar et al.patents mentioned above, and also incorporating the principles of thepresent invention. The reforming exchanger 100 has a tube side fluidinlet 102, shell side fluid inlet 104, and shell side fluid outlet 106in an elongated shell 108 having respective relatively high and lowtemperature ends 110 and 112, respectively. As illustrated, theelongated shell 108 can be cylindrical from the high temperature end(“first end”) 110 to the low temperature end (“second end”) 112. Theshell side fluid inlet 104 is adjacent to the high temperature end 110for receiving a hot gas feed. The tube side fluid inlet 102 is adjacentto the low temperature end 112 for receiving a feed mixture ofhydrocarbon and steam. The shell side fluid outlet 106 is fluidlyisolated from the tube side fluid inlet 102 by tube sheet 114 that isadjacent to the low temperature end 112 for discharging cooled gas fromthe reforming exchanger 100.

A tube bundle 116 is made up of a plurality of tubes 118 and one or morelongitudinally spaced transverse baffle plates 120. The tubes 118 havean inlet end 122 secured to the tube sheet 114 for receiving the gasmixture, and an outlet end 124 that is adjacent to the shell side fluidinlet 104 for discharging reformed gas into the hot gas feed. Lowpressure drop (ΔP) catalyst-bearing monolithic structures (see FIGS.2-7) are disposed within the tubes for converting the gas feed mixtureto reformed gas.

The tubes 118 preferably have a ratio of L_(t)/D_(t) of at least 300,more preferably at least 450. In determining L_(t)/D_(t), the diameterD_(t) refers to the inside diameter of the tubes 118 in the case ofright circular cylindrical tubes, or to the equivalent hydraulicdiameter in the case of non-circular tubes. The length L_(t) refers tothe catalyst-filled or -packed length. Higher L_(t)/D_(t) ratios arepreferred in the present invention because the heat transfercoefficients are generally higher than with a lower L_(t)/D_(t) ratio,and the resulting equipment cost is lower. A longer, smaller-ID catalysttube 118 generally results in more tubes 118 in the tube bundle 116, butthe tube bundle 116 has a smaller diameter for a given conversioncapacity, allowing the use of a shell 108 that has a smaller diameter.In general, the reduction of the diameter of the shell 108 and tubebundle 116 results in more capital cost savings than result from anyincrease in the length thereof, and thus the reforming exchanger 100 ofthe present invention can be much cheaper to fabricate than a prior artreforming exchanger of equivalent capacity. This result is particularlyadvantageous in the design of a new reforming exchanger 100.

Or, if it is desired to use the same shell diameters and tube lengths ofa prior art reforming exchanger so that the capital costs thereof aresubstantially equivalent, then the conversion capacity of the reformingexchanger 100 is substantially increased. This latter result isparticularly advantageous in the retrofitting of existing reformingexchangers by replacing the existing tube bundle with a tube bundle 116that has relatively more smaller-ID tubes 118 so that the retrofittedreforming exchanger 100 has a higher capacity than the originalreforming exchanger.

In the present invention, the ratio of the tube inside diameter (ID),D_(t), to the largest edge dimension of the catalyst structure (D_(p))can be relatively small compared to the same ratio in conventionalreforming exchangers. For example, in prior art reforming exchangersemploying Raschig ring catalyst measuring 0.31-in. OD by 0.125-in. ID by0.31-in. long, the minimum tube ID was about 2 inches. In the presentinvention, the same Raschig ring catalyst can be used in approximately1.25-in. or even 1-in. ID tubes with an equivalent or slightly higherratio of heat transfer to pressure drop. In the present invention, theD_(t)/D_(p) ratio is preferably not more than 4, and more preferablyabout 3 or less.

A low ΔP catalyst structure is defined herein as any suitable catalyststructure that results in a higher rate of heat transfer per unit oftube side pressure drop than in 2-in. ID reforming exchanger tubesfilled with catalyst-supporting Raschig rings measuring 0.31-in. OD by0.125-in. ID by 0.31-in. long under similar operating conditions andconversions.

Several different types of low ΔP monolithic catalyst support structuresare contemplated as being useful in the present invention. While the lowΔP is the most important property in the present invention, theexemplary catalysts are also typically found to have a relatively highvoid fraction and present a tortuous flow path to the tube side fluid.Catalyst activity can be relatively low to moderate without significantreduction in conversion rates, although there is no general detriment tousing a high activity catalyst aside from the typically higher costinvolved.

With reference to FIGS. 2A, 2B and 3, the catalyst support 200 is aribbed ring catalyst structure comprising an overall cylindrical shapewith a central longitudinal bore 202 and exterior ribs 204 runningparallel to a longitudinal axis. The depth of the V-shaped channel 206between the ribs is preferably from 0.1 to 0.3 times the OD of thesupport 200. Supports 200 measuring 0.2362-in. (6 mm) OD by 0.0787-in.(2 mm) ID by 0.2362-in. (6 mm) long with a fin 204 height of 0.0787-in.(2 mm) comprise one example of a suitably dimensioned support 200 foruse in nominal 1-inch or 1.5-inch tubes.

The ribbed ring supports 200 can be made by pressing a ceramic precursorinto molds with a pin to make the central bore 202, followed bycalcining the material at elevated temperatures, e.g. 2500° F., to forma ribbed ring support made of an α-alumina, for example, andimpregnating the α-alumina with nickel or another suitable catalyticallyactive material. Ribbed ring catalyst is commercially available, forexample, from Süd-Chemie Inc. of Louisville, Ky. Because of therelatively large size of the ribbed ring catalyst compared to the tube118 ID, the catalyst should preferably be loaded into the tubes 118using a dense loading method such as is accomplished with the equipmentand methodology described in U.S. Pat. Nos. 6,132,157, 5,897,282, and5,890,868, which are hereby incorporated herein by reference, in orderto minimize any packing or bridging problems.

With reference to FIG. 4, the catalyst insert 300 is in the form of atwisted tape having an OD about the same as the ID of the tube 118 inwhich it is used. The OD of the insert 300 is slightly less than the IDof the tube 118 to facilitate placement of the tape insert 300. Thelength of the insert 300 can be essentially the same length as the tubewith one insert 300 in each tube 118, or multiple inserts 300 can beplaced end-to-end in each tube 118. For the multiple inserts 300, eachinsert 300 preferably has a length which is at least as great as thediameter in order to keep the inserts 300 longitudinally aligned in thetube 118. The insert 300 can be made of a catalytically active materialsuch as nickel, or it can be coated with a catalytically activematerial. For example, the insert 300 can be wash coated with a ceramicas described in U.S. Pat. No. 5,980,843 to Silversand or U.S. Pat. No.5,935,889 to Murrell et al., both of which are hereby incorporatedherein by reference in their entireties, and the ceramic coatingimpregnated with a nickel catalyst by conventional ceramic impregnationtechniques. Catalytically inactive forms of such inserts 300 arecommercially available for increasing the tube side heat transfercoefficients in a shell-and-tube heat exchanger from, for example, theBrown Fintube Company of Houston, Tex.

With reference to FIG. 5, the catalyst insert 400 is in the form of abrush comprising a central runner 402 and a plurality of bristles orfilaments 404 extending transversely therefrom. The brush insert 400 hasan OD about the same as the ID of the tube 118 in which it is used. Thelength of the insert 400 can be essentially the same length as the tube118 with one insert 400 in each tube 118, or multiple inserts 400 can beplaced end-to-end in each tube 118, optionally with some overlap. Forthe multiple inserts 400, each insert 400 should have a length which isat least several times as great as the diameter in order to keep theinserts 400 longitudinally aligned in the tube 118. The insert 400 canbe made of a catalytically active material such as nickel, or it can becoated with a catalytically active material. For example, the insert 400can be wash coated with a ceramic as described above, and the ceramiccoating impregnated with a nickel catalyst by conventional ceramicimpregnation techniques. Catalytically inactive forms of such inserts400 are commercially available for increasing the tube side heattransfer coefficients in a shell-and-tube heat exchanger under, forexample, the trade designation HITRAN.

With reference to FIGS. 6A, 6B and 7, the catalyst insert 500 is in theform of a ceramic foam. The ceramic foam insert 500 is preferably madeby filling voids in an organic sponge substrate with a fluidized ceramicprecursor and burning the substrate away to form the ceramic foam. Theceramic foam can be impregnated with nickel or another catalyticallyactive material using conventional nickel impregnation techniques. Theceramic foam insert 500 is preferably made in sheets, plugs 502 are cutfrom the sheets having a diameter less than a thickness, and a pluralityof the plugs 502 are stacked end-to-end in each tube 118. If necessary,the sheet can be filled with liquid wax, which is solidified tofacilitate the cutting of the plugs 502, and then the wax is removed bymelting it. The plug 502 has an OD about the same as the ID of the tube118 in which it is used. The length of each plug 502 should be at leastas great as the ID of the tube 118 to help keep the plug 502 alignedinside the tube 118. The plugs 502 are placed end-to-end in each tube118 as illustrated in FIG. 7.

EXAMPLES

In the following examples, the tube side heat transfer coefficients,flow rates and pressure drops are based on a tube side inlet gas havingthe composition in Table 1:

TABLE 1 Component Mole Percent N₂ 0.23 H₂ 0.34 CH₄ 15.49 Ar <0.01 CO₂0.03 C₂H₆ 1.03 C₃H₈ 0.34 iC₄H₁₀ 0.10 iC₅H₁₂ 0.02 nC₆ 0.02 CO 0.00 H₂O82.40 Total 100.00

Examples 1-2

Conceptual sizing reviews were done on various types of catalyst sizesand tube inserts. The catalyst was of a normal size currently used inreforming exchangers available from Kellogg Brown & Root, Inc. under thetrade designation KRES (Raschig rings 0.31-in. OD by 0.125-in. ID by0.31-in. long), a smaller catalyst size (Raschig rings 0.25-in. OD by0.10-in. ID by 0.25-in. long), a smallest catalyst size (Raschig rings0.185-in. OD by 0.07-in. ID by 0.185-in. long), a twisted tape insertsuch as a Turbulator available from Brown Fintube but made of nickel 201(99.6% nickel), and a ceramic foam insert impregnated with nickel. Theresults are summarized in table 2:

TABLE 2 Base Comparative Comparative Comparative Parameter Case ExampleA Example B Example C Example 1 Example 2 Catalyst Normal SmallerSmallest Solid Twisted Ceramic (0.31 × 0.125 × 0.31) (0.25 × 0.10 ×0.25) (0.185 × 0.07 × 0.185) Pellets tape foam (0.185 × 0.185)Refractory 103 114 110 120 48 88 ID (in.) Tube 44 44 27 24 60 41 length(ft) Tube OD 2 2.5 1.5 1.5 0.75 1.15 (in.) Relative no. Base 0.79 2.032.33 1.61 1.62 of tubes Relative Base 0.986 0.936 0.954 0.822 0.833surface area Tube ΔP 28 29 28 27 10 29 (psi) Shell side 8 8 8 8 8 8 ΔP(psi) MTD (° F.) 145.5 153.2 140.7 135.9 139.8 150 Overall U 55.7 53.463.8 65.3 68.3 60.0 (Btu/hr-ft²-° F.) Approach 3.6 3.4 5.1 5.9 5.1 5.3to equilibrium (° F.) Estimated Base 1.06 0.96 0.99 0.37 0.48 cost(relative)

These results show little or no advantage in the use of the smaller orsmallest drilled cylinder catalyst shapes, or solid cylindrical pellets,Comparative Examples A, B and C, respectively. The smaller catalystsizes result in larger diameter reactors (refractory ID), assuming thatthe same allowable pressure drop is available, as in ComparativeExamples A, B and C. Although the tube lengths are shorter inComparative Examples B and C, the larger reactor diameters result inpremium costs and also present problems in tubesheet fabrication andquality control.

The designs with the twisted tape insert and the ceramic foam (Examples1 and 2) use smaller diameter tubes and lattice or egg crate typebaffles, resulting in longitudinal shell side flow and improved shellside performance. Combined with the enhanced tube side performance, thisresults in a more cost effective design with a lower pressure drop.Example 1 is based on the twisted insert measuring 0.625-in. wide by0.035-in. thick and twisted to 4 revolutions per foot. Performancesizing assumed the same targeted methane slip (2.5%) and the sameactivity factors as with conventional catalysts. Nickel impregnation ofa ceramic coating on the twisted tape insert can improve catalyticactivity.

Example 3

Conceptual sizing reviews were done as in Examples 1-2 to compare thepressure drop and performance of Raschig ring catalyst against ribbedring catalyst. Both catalyst structures measured 0.31-in. OD by0.125-in. ID by 0.31-in. long, and the V-shaped grooves between the ribson the ribbed ring catalyst were 0.17-in. deep. The results arepresented in Table 3:

TABLE 3 Comparative Parameter Example D Example 3 Catalyst 0.31 in. ×0.125 in. × 0.31 in. × 0.125 in. × 0.31 in. 0.31 in. Raschig ribbedrings rings Refractory ID 55 55 (in.) Tube length 25 25 (ft) Tube ID(in.) 2.00 2.00 Tube OD (in.) 2.25 2.25 Relative no. Base 1.0 of tubesRelative Base 1.0 catalyst volume Relative Base 1.0 surface area Tube ΔP(psi) 21.5 12.5 Shell side ΔP 8 8 (psi) MTD (° F.) 211 225 Overall U 5148 (Btu/hr-ft²-° F.) Approach to 33.4 29.1 equilibrium (° F.) Methaneslip 1.19 1.16 (%)

The data for Example 3 demonstrate that the performance of the ribbedring catalyst is generally equivalent to Raschig rings of the same size,except that the tube side pressure drop is substantially lower. The costof the ribbed ring reforming exchanger with a correspondingly reducednumber of relatively longer tubes would be much less since exchangeroverall length is generally less expensive than exchanger diameter.

Examples 4-5

Conceptual sizing reviews were done as in Examples 1-3 for various sizesof catalyst tube ID's (2.0, 1.55 and 1.00-in.) using standard Raschigring catalyst. The results are presented in Table 4:

TABLE 4 Comparative Parameter Example D Example 4 Example 5 Catalyst0.31 in. × 0.125 in. × 0.31 in. × 0.125 in. × 0.31 in. × 0.125 in. ×0.31 in. 0.31 in. 0.31 in. Raschig Raschig Raschig rings rings ringsRefractory 91.3 84.6 81.9 ID (in.) Tube 40 35 25 length (ft) Tube ID2.00 1.55 1.00 (in.) Tube OD 2.25 1.75 1.125 (in.) Relative no. Base1.43 2.91 of tubes Relative Base 0.75 0.45 catalyst volume Relative Base0.964 0.912 surface area Tube ΔP 35.5 34.5 33.8 (psi) MTD (° F.) 167.9157.9 150.2 Overall U 64.7 71.4 79.9 (Btu/hr-ft²-° F.) Approach 6.9 8.414.9 to equilibrium (° F.) Estimated Base 0.84 0.64 cost (relative)

The data for Examples 4 and 5 show, quite surprisingly, that employingsmaller tubes, i.e. a lower D_(t)/D_(p) ratio, using the conventionalRaschig rings, has the result of significantly reducing the catalystvolume and cost of the reforming exchanger, while maintaining the samecapacity.

Example 6

Tubes of various sizes were packed with catalyst shapes comprisingRaschig rings, ribbed rings and ceramic foam plugs in a laboratory tubeevaluation apparatus. Air was passed through the packed tubes atReynold's numbers similar to those seen in commercial reforming reactortubes. The tubes were externally heated to provide tube walltemperatures within a range expected in commercial reforming reactortubes. Heat transfer coefficients (Btu/hr−ft²−° F.) for the insidesurface of the tubes were determined and pressure drop (psi/ft) wasmeasured. The data were used to compare the ratio of heat transfer topressure drop relative to a 1.94-in. ID tube with Raschig ring catalystsupports. The ratio was determined for Raschig rings and ribbed rings in1.94-in. ID and 1.38-in. ID tubes, and for Raschig rings, ribbed ringsand ceramic foam in 1.05-in. ID tubes. The results are presented in FIG.8, and show that the relative ratio of heat transfer to tube sidepressure drop is significantly higher for ribbed ring catalyst at alltube diameters, and for ceramic foam catalyst at the smaller tubediameter tested.

The foregoing description and examples of the invention are merelyillustrative thereof. Various changes and modifications will be obviousto the skilled artisan in view of the foregoing disclosure. All suchvariations that fall within the scope or spirit of the appended claimsare intended to be embraced thereby.

1. An apparatus for reforming syngas, comprising: an elongated shellhaving relatively high and low temperature ends; a shell side fluidinlet adjacent the high temperature end; a tube side fluid inletadjacent the low temperature end; a shell side fluid outlet fluidlyisolated from the tube side fluid inlet by a tube sheet adjacent the lowtemperature end; a tube bundle comprising a plurality of tubes and oneor more longitudinally-spaced transverse baffle plates, wherein thetubes have an inlet end secured to the tube sheet and an outlet endadjacent the shell side fluid inlet; and catalyst-bearing particlesdisposed within the tubes, wherein the catalyst-bearing particlescomprise Raschig rings, ribbed rings, or a combination thereof, whereinthe tubes have an inside diameter that is not more than 4 times amaximum edge dimension of the catalyst-bearing particles, and whereinthe elongated shell contains no other catalyst other than thecatalyst-bearing particles disposed within the tubes.
 2. The apparatusof claim 1 wherein the tubes have an L_(t)/D_(t) ratio of at least 300wherein L_(t) is taken as the length of the catalyst bearing extent ofthe tubes and D_(t) is the inside diameter of the tubes.
 3. Theapparatus of claim 1 wherein the catalyst-bearing particles compriseRaschig rings.
 4. The apparatus of claim 3 wherein the inside diameterof the tubes is from 0.75 inches to 1.55 inches.
 5. The apparatus ofclaim 3 wherein the Raschig rings have an outer diameter of 0.31 inches,an inner diameter of 0.125 inches, and a length of 0.31 inches.
 6. Theapparatus of claim 1, wherein a surface area of the catalyst-bearingparticles for a given capacity is less than Raschig rings measuring 0.31inches long, by 0.31 inches outside diameter, by 0.125 inches insidediameter in tubes having an inside diameter of 2 inches.
 7. Theapparatus of claim 6 wherein a catalyst volume of the catalyst-bearingparticles for the given capacity is less than the Raschig rings in the 2inch inside diameter tubes.
 8. The apparatus of claim 1 wherein thecatalyst-bearing particles are disposed within the tubes in a packedarrangement.
 9. The apparatus of claim 1 wherein the tubes have anL_(t)/D_(t) ratio of at least 450 wherein L_(t) is taken as the lengthof the catalyst bearing extent of the tubes and D_(t) is the insidediameter of the tubes.
 10. The apparatus of claim 1 wherein an innersurface of the elongated shell is cylindrical and has a constantdiameter extending from the relatively high temperature end to therelatively low temperature end, and wherein the shell side fluid inlet,the tube side fluid inlet, and the shell side fluid outlet are alllocated between the relatively high temperature end and the relativelylow temperature end.
 11. The apparatus of claim 1 wherein thecatalyst-bearing particles comprise ribbed rings.
 12. The apparatus ofclaim 11 wherein the ribbed rings have an outer diameter of 0.2362inches, an inner diameter of 0.0787 inches, a length of 0.2362 inches,and a fin height of 0.0787 inches, and wherein the length of the ribbedrings is equal to between ⅓ and ¼ the inside diameter of the tubes. 13.The apparatus of claim 1, wherein the tubes have an inside diameter thatis not more than 3 times the maximum edge dimension of thecatalyst-bearing particles.
 14. The apparatus of claim 1 wherein thecatalyst-bearing particles are disposed within the tubes in a packedarrangement, wherein the catalyst-bearing particles comprise Raschigrings, and wherein the tubes have an inside diameter that is not morethan 3 times the maximum edge dimension of the catalyst-bearingparticles.
 15. The apparatus of claim 1 wherein the catalyst-bearingparticles are disposed within the tubes in a packed arrangement, whereinthe catalyst-bearing particles comprise Raschig rings, and wherein thetubes have an inside diameter that is not more than 3 times the maximumedge dimension of the catalyst-bearing particles, wherein the tubes havean L_(t)/D_(t) ratio of at least 300 wherein L_(t) is taken as thelength of the catalyst bearing extent of the tubes and D_(t) is theinside diameter of the tubes, and wherein the inside diameter of thetubes is from 0.75 inches to 1.55 inches.
 16. A reforming exchanger,comprising: a shell having one or more catalyst-containing tubesdisposed therein, wherein an inner surface of the shell is cylindricaland has a constant diameter extending from a first end to a second endthereof; a first fluid inlet in fluid communication with the first endof the shell; a second fluid inlet in fluid communication with thesecond end of the shell; a fluid outlet in fluid communication with thesecond end of the shell, wherein the first fluid inlet, the second fluidinlet, and the fluid outlet are all located between the first end andthe second end of the shell; a tube sheet disposed within the second endof the shell, the tube sheet adapted to isolate the fluid outlet fromthe second fluid inlet; and a tube bundle comprising the one or morecatalyst-containing tubes and a plurality of longitudinally-spacedtransverse baffle plates, wherein the tubes have an inlet secured to thetube sheet and an outlet adjacent the first fluid inlet, and wherein thetube bundle is disposed within the shell; wherein thecatalyst-containing tubes each comprise one or more catalyst-bearingparticles disposed therein, wherein the catalyst-bearing particles areselected from the group consisting of Raschig rings, ribbed rings, or acombination thereof, wherein the tubes have an inside diameter that isnot more than 4 times a maximum edge dimension of the catalyst-bearingparticles, and wherein the shell contains no other catalyst other thanthe catalyst-bearing particles disposed within the tubes.
 17. Theexchanger of claim 16, wherein the tubes have an L_(t)/D_(t) ratio of atleast 300 wherein L_(t) is taken as the length of the catalyst bearingextent of the tubes and D_(t) is the inside diameter of the tubes. 18.The exchanger of claim 16, wherein the inside diameter of the tubes isfrom 0.75 inches to 1.55 inches.
 19. The exchanger of claim 16, whereina surface area of the catalyst-bearing particles for a given capacity isless than Raschig rings measuring 0.31 inches long, by 0.31 inchesoutside diameter, by 0.125 inches inside diameter in tubes having aninside diameter of 2 inches.
 20. The exchanger of claim 19, wherein acatalyst volume of the catalyst-bearing particles for the given capacityis less than the Raschig rings in the 2 inch inside diameter tubes. 21.The exchanger of claim 16 wherein the catalyst-bearing particles aredisposed within the tubes in a packed arrangement, and wherein thecatalyst-bearing particles consist essentially of the Raschig rings andhave an outer diameter of 0.31 inches, an inner diameter of 0.125inches, and a length of 0.31 inches.
 22. The exchanger of claim 16wherein at least a portion of the plurality of longitudinally-spacedtransverse baffle plates define a transverse flow path across at least aportion of the tubes.
 23. The exchanger of claim 16 wherein the tubeshave an L_(t)/D_(t) ratio of at least 450 wherein L_(t) is taken as thelength of the catalyst bearing extent of the tubes and D_(t) is theinside diameter of the tubes.
 24. An apparatus for reforming syngas,comprising: a shell having relatively high and low temperature ends; ashell side fluid inlet adjacent the high temperature end; a tube sidefluid inlet adjacent the low temperature end; a shell side fluid outletfluidly isolated from the tube side fluid inlet by a tube sheet adjacentthe low temperature end; a tube bundle disposed within the shell, thetube bundle comprising a plurality of tubes and a plurality oflongitudinally-spaced transverse baffle plates, wherein at least aportion of the plurality of longitudinally-spaced transverse baffleplates define a transverse flow path across at least a portion of thetubes, wherein the tubes have an inlet end secured to the tube sheet andan outlet end adjacent the shell side fluid inlet, and wherein the tubeshave an L_(t)/D_(t) ratio of at least 300 wherein L_(t) is taken as thelength of the catalyst bearing extent of the tubes and D_(t) is theinside diameter of the tubes; and catalyst-bearing particles disposedwithin the tubes, wherein the catalyst-bearing particles compriseRaschig rings or ribbed rings, wherein the tubes have an inside diameterthat is not more than 4 times a maximum edge dimension of thecatalyst-bearing particles, and wherein the shell contains no othercatalyst other than the catalyst-bearing particles disposed within thetubes.
 25. The apparatus of claim 24 wherein the catalyst-bearingparticles are disposed within the tubes in a packed arrangement.
 26. Theapparatus of claim 24 wherein an inner surface of the shell iscylindrical and has a constant diameter extending from the relativelyhigh temperature end to the relatively low temperature end, and whereinthe shell side fluid inlet, the tube side fluid inlet, and the shellside fluid outlet are all between the relatively high and lowtemperature ends.
 27. The reforming exchanger of claim 24 wherein thecatalyst-bearing particles comprise Raschig rings and have an outerdiameter of 0.31 inches, an inner diameter of 0.125 inches, and a lengthof 0.31 inches.
 28. The apparatus of claim 22, wherein the elongatedshell is a cylinder, and wherein the tubes have an inside diameter thatis not more than 3 times the maximum edge dimension of thecatalyst-bearing particles.
 29. The apparatus of claim 24 wherein thecatalyst-bearing particles are disposed within the tubes in a packedarrangement, wherein the catalyst-bearing particles comprise Raschigrings, and wherein the tubes have an inside diameter that is not morethan 3 times the maximum edge dimension of the catalyst-bearingparticles.
 30. The apparatus of claim 24 wherein the catalyst-bearingparticles are disposed within the tubes in a packed arrangement, whereinthe catalyst-bearing particles comprise Raschig rings, wherein the tubeshave an inside diameter that is not more than 3 times the maximum edgedimension of the catalyst-bearing particles, wherein the tubes have anL_(t)/D_(t) ratio of at least 450 wherein L_(t) is taken as the lengthof the catalyst bearing extent of the tubes and D_(t) is the insidediameter of the tubes, and wherein the inside diameter of the tubes isfrom 0.75 inches to 1.55 inches.
 31. An apparatus for reforming syngas,comprising: an elongated shell having a first elliptical head disposedon a first end thereof, a second elliptical head disposed on a secondend thereof, and a cylindrical inner surface having a constant diameterextending from the first end to the second end; a tube sheet disposedwithin the elongated shell between the first and second ends thereof toprovide a first internal volume toward the first end of the elongatedshell and a second internal volume toward the second end of theelongated shell; a fluid outlet and a first fluid inlet each in fluidcommunication with the first internal volume at locations intermediatethe first and second ends of the elongated shell; a second fluid inletin fluid communication with the second internal volume at a locationintermediate the first and second ends of the elongated shell; a tubebundle comprising a plurality of tubes and a plurality oflongitudinally-spaced transverse baffle plates, wherein the tubes havean inlet end secured to the tube sheet and in fluid communication withthe second fluid inlet via the second internal volume and an outlet endadjacent the first fluid inlet and in fluid communication with the firstinternal volume, and wherein at least a portion of the plurality oflongitudinally-spaced transverse baffle plates define a transverse flowpath across at least a portion of the tubes; and catalyst-bearingparticles consisting of Raschig rings disposed within the tubes, whereinthe tubes have an inside diameter that is not more than 4 times amaximum edge dimension of the catalyst-bearing particles, wherein theelongated shell contains no other catalyst other than thecatalyst-bearing particles disposed within the tubes, and wherein thetubes have an L_(t)/D_(t) ratio of at least 450, where L_(t) is taken asthe length of the catalyst bearing extent of the tubes and D_(t) is theinside diameter of the tubes.
 32. The apparatus of claim 31 wherein theelongated shell is a cylinder, wherein the Raschig rings have an outerdiameter of 0.31 inches, an inner diameter of 0.125 inches, and a lengthof 0.31 inches, wherein the Raschig rings are disposed within the tubesin a packed arrangement, and wherein the tubes have an inside diameterthat is not more than 3 times a maximum edge dimension of thecatalyst-bearing particles.
 33. The apparatus of claim 31 wherein thecatalyst-bearing particles are disposed within the tubes in a packedarrangement.
 34. The apparatus of claim 31 wherein the catalyst-bearingparticles are disposed within the tubes in a packed arrangement, andwherein the tubes have an inside diameter that is not more than 3 timesthe maximum edge dimension of the catalyst-bearing particles.